1. Field of the Invention
The present invention relates to voltage converters, in general, and in particular to converters that provide DC voltages to digital devices.
2. Description of the Related Art
The majority of electrical devices, especially digital ones, require DC voltages for proper operation. As these devices become more sophisticated both in functions and operation more demands are placed on power supplies or power rectifiers (hereafter called Power Conversion Units (or PCUs) to provide a more controlled DC voltage. Among the demands are the requirements that the DC voltage contains acceptable ripple and be available for a predefined time interval (hereafter termed holdup).
The current state of the art uses a PCU to convert service AC voltage to either a distributed DC output voltage or server component level DC voltage. The PCUs can be broadly classified into two groups. The groupings are based upon the topology of the PCUs. The PCUs in the first group are designed with a two stage topology. Even though the two stage PCUs work well for intended purposes they have drawbacks. In particular, the efficiency of the two stage PCUs is believed to be lower than what the electrical industry would like to have. The efficiency of such PCUs is a multiplication of the efficiency of the two stages. For example, if both stages were 95% efficient, the efficiency of a PCU in this group would be 90%. This level of efficiency is well below what is expected from a well designed PCU. The PCUs in the second group are designed with a single stage topology which is believed to be more efficient than the two stage topology. But even with the improved efficiency, the single stage PCUs have drawbacks too. One of the drawbacks is that the single stage PCUs generate higher, than acceptable, output ripple voltages. This higher output ripple voltage is unacceptable in many applications. As a consequence, the single stage topology would not be suitable for these applications. Another drawback is that some single stage topology PCUs are unable to holdup their output voltage for a predefined time interval. This is unacceptable because many power supply users require that the PCU holds the output DC voltage for a predefined time interval, usually between 10 and 20 milliseconds. Again the single stage topology is not suited for these applications.
Still another drawback is that the frequency response of a single stage converter is often insufficient for the load, causing large dips and overshoots in the output voltage due to dynamic change in the load.
In view of the above, there is a need for a PCU that is free from the shortcomings set forth above. Such a PCU is also needed to provide service to the applications that could not use a PCU with a single stage topology.